Wells, such as those used for oil and gas extraction, comprise a bore extending into the seabed or ground, and lined with an internal well casing preventing any fluid extracted from coming into contact with ground or sea water. A well casing comprises a series of tubes, known as internal casing strings, with the diameter of each successive string being smaller than the last as the depth of the well increases, enabling drilling of the bore in stages. Once a portion of the bore is drilled, the casing string is inserted into the bore, with the annular space between the casing and the inner surface of the bore being filled with cement to hold the casing in place. Since successive casing strings must be passed through those already in place, they necessarily have a smaller diameter than the preceding casing. Consequently the bore is lined with multiple casing strings.
Once a well reaches the end of its economic life it is decommissioned, with the well needing to be plugged and abandoned and the casing strings recovered. This is done by applying a permanent (or occasionally temporary) plug and cutting or severing the casing string. Cutting the string requires deploying a downhole cutting tool within the casing and cutting the casing, for example, using an abrasive slurry jet, and removing the tool once the string is cut. Multiple string cutting tools are typically concentric with the casing string, and lowered down from the surface to be held in position whilst cutting takes place. Examples of such tools are disclosed in GB2,354,726 and GB2,463,849, for example.
Existing tools use an inflatable packer as part of the tool, which is inflated at the deployment depth (the depth at which the string needs to be cut) to restrain the device creating a seal within a well casing and preventing axial movement of the tool away from the deployment depth. Pressurised air is pumped below the tool to enable “de-watering”, evacuating water from the target region of the abrasive slurry jet for example, which ensures the quality of abrasive cutting. One issue is that the pressure below the tool creates an upwards acting force on the tool, which has been known to cause the tool to slip upon failure of the inflatable packer. This can cause issues either with the cutting operation or damage or potential loss of the tool itself (which may fall downwards or be expelled from the well).
There is therefore a need to be able to deploy and recover multiple string cutting tools safely, reliably and in such a manner that the likelihood of the tool becoming damaged or lost, or the cutting operation being affected detrimentally, is minimised.